How do I permanently disable the security alarm system on my 1997 Nissan Quest?

I've had this van for 20 years.  Recently, the security alarm goes off for no
reason as my van sits in my garage locked.  If I'm not home to stop it, the
battery runs dead.  Can you tell me how to permanently disable the security
system?  I would rather not have it at all, than to have my battery keep going
dead.